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On 25 Apr 2007 at 18:17, Alexandre E Eremenko wrote:

> Let me add few comments of my own.
> I am really interested in this question whether
> the best XIX century sextants (made for the Lunars)
> were better than out modern ones.

Peter Fidler in 1791, using a "brass sextant of five or six inches radius 
made by Ramsden" reports the following two latitudes (among many celestial 
observations, but I'm pretty sure that I know where he was when he made 
these two).

51�50'47" (actual latitude 51�02'55")
52�21'37" (actual latitude 52�15'35")

These are very bad but Fidler was a beginner at the time. Even still, I 
have plotted his course on this trip and his lunars are not nearly as bad 
as one would expect if his readings were this awful. I'll check tonight to 
see if there is some better data to evaluate this small Ramsden sextant.

David Thompson used a 10 inch radius brass sextant made by Dollond. His 
latitudes at Rocky Mountain House from 1800 and 1801 were:

52�21'29"
52�21'27"
52�21'35"
52�21'32"

The actual latitude is 52�21'20" but it should be remembered that Rocky 
Mountain House is at an altitude of 3200' and Thompson did not account for 
that in his calculation. So his sextant certainly seems pretty good at 
this part of the arc. Recall that I posted his lunars from this same 
position about 2 years ago and after correcting for the poor data in the 
almanac, his sigma was about 14' in longitude for 13 observations. Not 
that great, but it's hard to know how much was due to his sextant and how 
much to observational errors.

Ken Muldrew.

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